During pregnancy, the placenta of the mare secretes a glycoprotein hormone, pregnant mare serum gonadotropin (PMSG), which has the unique property of possessing the biological actions of the two pituitary gonadotropins, follicle stimulating hormone (FSH) and the interstitial cell-stimulating hormone (ICSH, LH). Therefore, the study of PMSG will give us information on both FSH and ICSH (LH). The proposed research on PMSG is related to the ultimate goal of gaining a better understanding of the function and mechanism of gonadotropin action as it relates to reproduction. The techniques of protein and polypeptide chemistry will be applied to obtain highly purified PMSG such that structural analysis, immunochemical, and biological studies can be meaningfully performed. Specific bioassays will be employed to correlate the structural elements encountered to biological activity. It is anticipated that a detailed knowledge of the structure-function correlates of PMSG will lead to a greater insight into the mechanism of gonadotropin action on the molecular level.